Method for generating photons by sonoluminescence

ABSTRACT

A method of generating photons by sonoluminescence, from a gas bubble trapped in a liquid reservoir ( 2 ) by a standing ultrasound wave. An ultrasound impulse emitted by high-frequency transducers (T 1 -T 8 ) is superposed on the standing wave, the high-frequency transducers being pre-focused onto the gas bubble and pre-synchronized with the light emissions from the gas bubble during an initial training stage in which said focusing and said synchronization are optimized.

[0001] The present invention relates to methods of generating photons bysonoluminescence.

[0002] More particularly, the invention relates to a method ofgenerating photons by sonoluminescence, said method comprising at leastthe following steps:

[0003] (a) generating at least one standing acoustic wave in a liquidreservoir, said standing acoustic wave having at least one antinode;

[0004] (b) trapping at least one gas bubble in the liquid at saidantinode of the standing acoustic wave, said gas bubble then beingsubjected to a periodic deformation cycle comprising expansion stagesand contraction stages in alternation; and

[0005] (e) generating acoustic wave impulses in the liquid, whichcompression acoustic wave impulses are superposed on the standingacoustic wave, and cause photons to be emitted by the gas bubble, bysonoluminescence.

[0006] A method of this type is described by Moss et al., who performeddigital simulations of the behavior of a deuterium bubble under theaction of a sinusoidal standing acoustic wave on which an acoustic waveimpulse was superposed (“Sonoluminescence and the prospects fortable-top micro-thermonuclear fusion”, Physics Letters A 211 (1996)69-74, Elsevier North Holland, Feb. 5, 1996). According to Moss et al.,the acoustic wave impulse superposed on the sinusoidal wave makes itpossible to increase photon emission by the gas bubble, and could evenlead to conditions making it possible to trigger a thermonuclear fusionreaction.

[0007] However, those theoretical results prove impossible to implementwith compression wave impulses having large amplitudes because it isthen impossible to apply to the gas bubble compression acoustic waveshaving amplitudes greater than 1.4 bars. Otherwise the gas bubble“disintegrates” and disappears without generating any photons.

[0008] A particular object of the present invention is to mitigate thosedrawbacks so that, when necessary, it is possible to apply to the gasbubble acoustic waves of amplitude significantly greater than 1.4 bars.

[0009] More generally, an object of the invention is to provide a methodof the above-mentioned type that makes it possible to amplify thephenomenon of sonoluminescence.

[0010] To this end, according to the invention, a method of the type inquestion is characterized:

[0011] in that the acoustic wave impulses are caused to be emitted by anumber n of not less than 2 impulse firing transducers disposed aroundthe gas bubble

[0012] in that a focusing training step (c) and a synchronization step(d) are interposed between the steps (b) and (e); and

[0013] in that, during the focusing training step (c), the impulsefiring transducers are caused to emit acoustic wave impulses with afirst amplitude that is sufficiently small to avoid disturbingsignificantly the position and the deformation cycle of the gas bubble,acoustic signals generated by said acoustic wave impulses in the liquidreservoir are measured, and time offsets are deduced therefrom to beapplied to respective ones of the acoustic wave impulses generated bythe various impulse firing transducers so as to focus said acoustic waveimpulses onto the gas bubble;

[0014] in that, during the step (d), instants at which acoustic waveimpulses are emitted by the various impulse firing transducers aredetermined so that each wave impulse generated by the impulse firingtransducers reaches the gas bubble either during a contraction stage ifthe wave impulse is a compression wave, or during an expansion stage ifthe wave impulse is an expansion wave; and

[0015] in that, during the step (e) the impulse firing transducers arecaused to generate the acoustic wave impulses at the respective emitinstants determined at the step (d), with a second amplitude that islarger than the first amplitude.

[0016] By means of these provisions, it is thus possible to obtain asignificant increase in the energy emitted by the gas bubble in the formof photons, for any given energy of the acoustic wave impulse. Inaddition, the second amplitude of the acoustic wave impulse mayoptionally be very much greater than 1.4 bars without destroying the gasbubble before it emits photons.

[0017] In preferred embodiments of the invention, any of the followingprovisions may optionally be used:

[0018] the step (c) comprises the following sub-steps:

[0019] (c1) each impulse firing transducer is caused to emit an acousticwave impulse in succession, with said first amplitude;

[0020] (c2) after each acoustic wave impulse emission, each impulsefiring transducer is caused to measure the acoustic signals s3_(ij)(t)generated by said acoustic wave impulse propagating in the liquidreservoir, and said measured signals are stored, i and j being indicesrespectively designating the impulse firing transducer that emitted theacoustic wave impulse and the impulse firing transducer that receivedthe acoustic wave impulse corresponding to each measured signals3_(ij)(t) ; and

[0021] (c3) at least on the basis of said measured signals s3_(ij)(t) ,said time offsets to be applied to respective ones of the acoustic waveimpulses generated by the various impulse firing transducers aredetermined so as to focus said acoustic wave impulses onto the gasbubble;

[0022] during the sub-step (c3), travel times taken by the acoustic waveimpulses to travel between each impulse firing transducer and the gasbubble are determined, and said time offsets to be applied to respectiveones of the acoustic waves generated by the various impulse firingtransducers so as to focus said acoustic wave impulses onto the gasbubble are deduced from said travel times;

[0023] a preliminary calibration step (a0) is performed, at least beforethe step (b), said calibration step comprising the following sub-steps:

[0024] (a01) each impulse firing transducer is caused to emit anacoustic wave impulse in succession, with said first amplitude;

[0025] (a02) after each acoustic wave impulse emission, each impulsefiring transducer is caused to measure acoustic signals s1_(ij)(t)generated by said acoustic wave impulse propagating in the liquidreservoir, and said measured signals s1_(ij)(t) are stored;

[0026] during step (c) each impulse firing transducer is caused tolisten to the acoustic signals s2_(j)(t) received while the standingacoustic wave is being emitted in the presence of the gas bubble;

[0027] and during the sub-step (c3), corrected signalss_(ij)(t)=s3_(ij)(t)−s1_(ij)(t)−s2_(j)(t) are calculated, and then saidtime offsets are determined on the basis of said corrected signals;

[0028] said time offsets are determined by cross-correlation betweensaid corrected signals;

[0029] n is at least equal to 8;

[0030] the acoustic wave impulses are compression acoustic waveimpulses, and, during the step (d), emit instants are determined atwhich the compression acoustic wave impulses are emitted by the variousimpulse firing transducers so that each compression acoustic waveimpulse generated by the impulse firing transducers reaches the gasbubble during a contraction stage;

[0031] during step (d), emission of the compression acoustic waveimpulses by the various impulse firing transducers is synchronized withthe deformation cycle followed by the gas bubble so that saidcompression acoustic wave impulses generate an increase in the pressureof the liquid surrounding the gas bubble at least until the end of saidcontraction stage;

[0032] during step (d), emission of the compression acoustic waveimpulses by the various impulse firing transducers is synchronized withthe deformation cycle followed by the gas bubble, so that eachcompression acoustic wave impulse generated by the impulse firingtransducers reaches the gas bubble substantially when said gas bubblehas its maximum diameter;

[0033] the compression acoustic waves generate acoustic vibration ofamplitude at least equal to 8 bars in the liquid in the vicinity of thegas bubble;

[0034] during step (e), the compression acoustic wave impulse comingfrom each impulse firing transducer is caused to be preceded immediatelyby an expansion acoustic wave impulse which is adapted to reach the gasbubble during the expansion stage preceding the contraction stage duringwhich said gas bubble receives the compression acoustic wave impulses;

[0035] the standing acoustic wave is caused to be generated by at leasttwo standing wave generation transducers distinct from the impulsefiring transducers; and

[0036] the standing acoustic wave is an ultrasound wave of frequencylying in the range 20 kilohertz (kHz) to 30 kHz and of amplitude in thevicinity of 1.3 bars.

[0037] Other characteristics and advantages of the invention will appearon reading the following description of one of its implementations,given by way of non-limiting example, and with reference to theaccompanying drawings.

IN THE DRAWINGS

[0038]FIG. 1 is a perspective view of an embodiment of the device thatmakes it possible to implement the method of the invention;

[0039]FIG. 2 is a plan view of the device of FIG. 1;

[0040]FIG. 3 is a section view on line III-III of FIG. 2;

[0041]FIG. 4 is a block diagram showing the control system of the deviceof FIGS. 1 to 3; and

[0042]FIG. 5 shows firstly the diameter P of the gas bubble contained inthe device of FIGS. 1 to 3, over time t, and secondly the pressure Pgenerated by the standing acoustic wave S1 and by the acoustic waveimpulses S2 over time t in the vicinity of the gas bubble (the curvesshown in FIG. 5 are not to scale and they merely give a diagrammaticindication of the variations in the diameter D and in the pressures Pover time)

[0043] In the various figures, like references designate elements thatare identical or similar.

[0044] FIGS. 1 to 3 show an example of a sonoluminescence device 1 whichincludes a reservoir 2 such as a spherical glass flask filled with waterand around which two low-frequency ultrasound transducers T′1 and T′2and eight high-frequency ultrasound transducers (or “impulse firingtransducers”) T1-T8 are disposed, as is a photomultiplier PH.

[0045] In the example shown, the eight high-frequency ultrasoundtransducers T1-T8 are supported by a rigid metal frame 3 which alsosupports the reservoir 2.

[0046] Advantageously, the various transducers T1-T8, T′1, T′2 areengaged in leaktight manner in orifices 2 a provided in the wall of thereservoir 2 (see FIG. 3) so as to optimize the efficiency of saidtransducers.

[0047] The two low-frequency transducers T′1, T′2 are piezoelectrictransducers adapted to emit a sinusoidal ultrasound wave at a frequencylying in the range 20 kHz to 30 kHz, and advantageously at about 25 kHz.

[0048] In this example, the reservoir 2 is of substantially sphericalshape and is provided with a top neck 2 b centered on a vertical axis Z.The reservoir 2 has a diameter D such that it forms a resonant cavitytuned to the frequency of the low-frequency transducers T′1, T′2.Naturally, the reservoir 2 could be of a different shape, e.g. ofcylindrical shape or of rectangular block shape.

[0049] In the example considered, the diameter D of the reservoir 2 may,for example, be equal to 6 centimeters (cm), so that the center of thereservoir 2 forms an antinode for the standing acoustic wave generatedby the transducers T′1 and T′2.

[0050] Thus, when the reservoir 2 is filled with liquid, e.g. degassedwater, and when a gas bubble is injected into the reservoir 2 in thevicinity of its center (e.g. by means of a syringe), said bubble 5remains trapped at the antinode, i.e. at the center of the reservoir(see FIG. 3). For example, the gas contained in the bubble 5 may be air,or deuterium, or else some other gas.

[0051] Preferably, the transducers T′1, T′2 generate a standing acousticwave that has an amplitude sufficient for the gas bubble 5 to emitphotons cyclically by sonoluminescence, as explained below.

[0052] The light intensity of the photon emissions can be measured bymeans of the photomultiplier PH.

[0053] Furthermore, the eight high-frequency acoustic transducers T1-T8are distributed three-dimensionally around the center of the reservoir2, i.e. around the position of the gas bubble 5, and they are directedtowards said gas bubble.

[0054] In the example in question, the eight high-frequency transducersare disposed in two vertical planes which are perpendicular to eachother, and each of which contains the axis Z, each of the two plansbeing disposed at 45° relative to the common direction of thelow-frequency transducers T′1, T′2.

[0055] In addition, also in the example in question, the high-frequencytransducers T1-T8 are disposed in pairs of diametrically oppositehigh-frequency transducers, the common direction of each pair ofdiametrically opposite high-frequency transducers T1-T2, T3-T4, T5-T6,T7-T8 being disposed at 45° relative to the axis Z.

[0056] In addition, the common axis of each pair of diametricallyopposite high-frequency transducers is perpendicular to the common axisof the other pair of diametrically opposite high-frequency transducersbelonging to the same vertical plane.

[0057] Naturally, other configurations may be adopted for thehigh-frequency transducers T1-T8 provided that they are distributedthree-dimensionally around the gas bubble 5. Furthermore, the number ofhigh-frequency transducers could be different from eight. Thus, it ispossible to implement the present invention with at least twodiametrically opposite high-frequency transducers, or with at least fourhigh-frequency transducers angularly positioned to converge on the gasbubble 5. In addition, the number of high-frequency transducers couldalso be greater than eight.

[0058] The high-frequency transducers used in the example consideredherein are piezoelectric transducers emitting at a frequency of 700 kHz,and they are adapted to emit a compression acoustic wave impulse.

[0059] As shown in FIG. 4, the various transducers T1-T8, T′1, T′2 areconnected to a control device which, in the example considered herein,comprises an electronics rack 6 that may itself be controlled by amicrocomputer 9. The electronics rack 6 includes an electronic centralprocessing unit (CPU) 7 associated with a central memory (M) 8, andadapted to control the low-frequency transducers T′1, T′2 and to receivethe measurements from the photomultiplier PH (the central processingunit 7 can then use its internal clock or an external clock to determinethe instants at which the light flashes are emitted by the gas bubble 5while sonoluminescence is taking place). In addition, the centralprocessing unit 7 is also connected, via buffer memories M1-M8, to thehigh-frequency transducers T1-T8 so as to be able to control saidtransducers so that the signals picked up by said transducers while theyare operating in receive mode can be stored in the memories M1-M8.

[0060] The above-described device operates as follows.

[0061] Firstly, before the gas bubble 5 is injected into the reservoir2, an initial calibration step is performed, preferably while thelow-frequency transducers T′1, T′2 are in operation. During thiscalibration step, each of the high-frequency transducers T1-T8 insuccession is caused to emit a compression acoustic wave impulse thathas a first amplitude that is relatively small (e.g. about 30kilopascals (kPa)), and the resulting acoustic signal is measured by allof the high-frequency transducers T1-T8. The 64 signals s1_(ij)(t)measured in this way are stored in memories M1-M8 (the indices idesignate respective transducers that have emitted respective ones ofthe wave impulses, and the indices j designate respective transducersthat have received respective ones of the wave impulses)

[0062] Subsequently, the gas bubble 5 is injected into the reservoir 2while the low-frequency transducers T′1, T′2 continue to generate theabove-mentioned low-frequency standing acoustic wave, which results in asinusoidal pressure signal S1 at the bubble 5, as shown in FIG. 5. Underthe effect of the standing wave, the gas bubble 5 is stabilized at thecenter of the reservoir 2 so that said bubble is substantiallyequidistant from the various transducers T1-T8, T′1, T′2 in the examplein question.

[0063] The amplitude of said standing wave is preferably about 1.3 barsin the liquid in the vicinity of the gas bubble 5, so that the gasbubble 5 emits photons by sonoluminescence, because of the deformationsthat are imposed on it by the standing acoustic wave. As shown in FIG.5, the diameter D of the gas bubble 5 varies considerably between amaximum diameter D1 which may, for example, lie approximately in therange 30 micrometers (μm) to 80 μm for a diameter at rest D0 of about 3μm, and a minimum diameter D2 which may be about 0.5 μm in the examplein question.

[0064] These variations in diameter take place in periodic cycles havingthe same period T as the standing sinusoidal wave S1 with expansionstages 10 in which the curve D(t) rises to a peak 11 alternating withcontraction stages 12 in which the curve D(t) falls suddenly to a cusp13 at the minimum diameter D2, which cusp is generally followed by acertain number of bounces 14 before the beginning of the followingexpansion stage 10.

[0065] The gas contained in the bubble 5 emits photons at the end ofeach contraction stage 13, during a very short period, e.g. lying in therange 10 picoseconds (ps) to 300 ps, depending on circumstances, underthe effect of the sudden densification of the matter inside the bubble5, and of the concomitant increase in the temperature and in thepressure inside said gas bubble 5, which increase in temperature and inpressure is sufficient to generate a plasma temporarily in the gasbubble. These light flashes are emitted by the gas bubble 5 with exactregularity, with the same period T as the standing acoustic wave.

[0066] Once the gas bubble 5 is stabilized at the center of thereservoir 2, a focusing training step is performed.

[0067] During this focusing training step, each transducer Ti is causedto measure the signal s2_(i)(t) that it receives due to the standingacoustic waves in the presence of the gas bubble 5.

[0068] This measurement is effected in a time slot of duration T0 andstarting, for each transducer Ti, at an instant t0i=t0+Δτ_(i)0, where:

[0069] t0 is an instant at which the diameter of the gas bubble is atits maximum (this instant is known by measuring the instants t1 at whichthe light flashes are emitted by the gas bubble 5, and by the shape ofthe curve D(t) which is known in advance) ; and

[0070] Δτ_(i)0 is a first approximation of the travel time taken by theacoustic wave impulse to travel between the transducer Ti and the gasbubble 5 (in the example in question, in which the reservoir 2 isspherical and the bubble 5 is at the center of the sphere of radius R,all of the times Δτ_(i)0 may be taken to be equal to R/c, where c is thespeed of the acoustic wave in the liquid that fills the reservoir 2).

[0071] After each listening period during which a transducer Ti listens,the same transducer Ti is caused to emit a compression wave impulse S2(see FIG. 5) with said first amplitude, which is sufficiently low (e.g.30 kPa) to avoid displacing the bubble 5 or interfering with its cyclicdeformations. This emission is performed such that the wave impulsereaches the gas bubble 5 at an instant at which its diameter D is at itsmaximum, so as to maximize the return signal reflected by the gas bubble(in other words, the transducer Ti emits its acoustic wave impulse at aninstant t0+k.T−Δτ_(i)0, where k is a non-zero natural integer [forexample, k may be equal to 2] and T is the period of the standingacoustic wave).

[0072] The resulting acoustic signal s3_(ij)(t) is then measured at eachhigh-frequency transducer Tj during a time slot of duration T0 andstarting at the instant t0′i=t0i+k.T, and the measured signal s3_(ij)(t)is stored in the corresponding buffer memory Mj.

[0073] The same signal measurements s2_(i)(t), s3_(ij)(t) are effectedsuccessively for all of the transducers Ti, while leaving sufficientrelaxation time Tr between each acoustic wave impulse fired by atransducer Ti and the measurement of the signal s2_(i+i)(t) for thefollowing transducer Ti+1.

[0074] For example, the relaxation time Tr may be about 150 milliseconds(ms) in the example in question, or, more generally greater than orequal to Q.T, where Q is the quality factor of the acoustic cavityformed by the reservoir 2 filled with liquid, and T is the period of thestanding acoustic waves.

[0075] The various signals s2_(i)(t), s3_(ij)(t) all have the sameduration T0 and are in phase relative to the reflection or to theemission of the acoustic waves by the gas bubble 5. For reasons ofsimplification, t may be taken to be equal to 0 at the beginning of eachtime slot, so that all of the signals s2_(i)(t), s3_(ij)(t) start at t=0and end at t=T0.

[0076] In order to be consistent with the signals s2_(i)(t) s3_(ij)(t) ,the signals s1_(ij)(t) measured previously may also be measured in timeslots starting 2.Δτ_(i)0 after the acoustic wave impulse has beenemitted by each transducer Ti, and by convention, t may also be taken tobe equal to 0 at the beginning of each time slot, so that all of thesignals s1_(ij)(t) start at t=0 and end at t=T0.

[0077] When all of the acoustic signals have been measured, 64 timesignals s3_(ij)(t) are obtained, from which the previously measured timesignals s1_(ij)(t) and s2_(j)(t) are subtracted, so as to obtain thesignals s_(ij)(t)=s3_(ij)(t)−s1_(ij)(t)−s2_(j)(t), where i is the indexof the transducer Ti that emits the signal and j is the index of thetransducer that receives the signal.

[0078] Naturally, it is possible firstly to measure all of the signalss2_(i)(t) simultaneously or almost simultaneously in time slotsbeginning at instants t0i=t0+Δτ_(i)0 (the instants ti0 may optionallycoincide if all of the transducers are equidistant from the bubble) andof duration T0, and then to measure the signals s3_(ij)(t) successivelyin time slots of duration T0 and starting at t0i+k1.T, t0i+k2.T,t0i+k3.T, etc.

[0079] Once the signals s_(ij)(t) have been determined, across-correlation method or any other known method is used to deducefrom them the exact travel time Δτ_(i) taken by the ultrasound waveimpulses to travel between each high-frequency transducer Ti and the gasbubble 5, for the specific position occupied by the gas bubble.

[0080] By way of non-limiting example, it is possible to calculate thefollowing cross-correlation functions:

c _(ij)(τ)=∫s_(ij)(t)*S_(r)(τ+t)dt

[0081] where i and j are indices lying in the range 1 to 8 in theexample in question, designating the transducers T1-T8, and s_(r)(t) isa reference time signal, corresponding for example, to the transceiverresponse of an impulse firing transducer.

[0082] By way of example, it is possible to measure s_(r)(t) once andfor all by causing an acoustic impulse to be fired by one of thetransducers T1-T8 into the liquid in the absence of any bubble 5, and bycausing the acoustic signal received by the diametrically oppositetransducer T1-T8 to be measured, said signal being multiplied by −1 toconstitute the reference signal s_(r)(t). At the same time, the time ΔTbetween the beginning of the emission of the acoustic impulse by thefirst transducer and the beginning of its reception by the diametricallyopposite transducer is also measured.

[0083] After the cross-correlation functions C_(ij)(τ) have beencalculated, the values of τ_(ij) that respectively maximize saidfunctions are determined (this optimization may, for example, beconsidered to be performed when each of the cross-correlation functionshas a value greater than 0.8), then the above-mentioned travel timesΔτ_(i) are calculated, e.g. by the following formula:${\Delta \quad \tau_{i}} = {\frac{1}{M}{\sum\limits_{j = 1}^{M}{\frac{( {\tau_{\quad {i\quad j}} + \tau_{j\quad i} - \tau_{j\quad j}} )}{2}\Delta \quad t}}}$

[0084] where M is the number of transducers taken into account in saidcalculation (certain transducers are generally omitted from saidcalculation, in particular those that are situated facing each other,and those for which the direct wave arrives at the same time as thediffused wave).

[0085] Optionally, it is possible simplify said calculation by using thefollowing formula: Δτ_(i)=τ_(ii)/2+ΔT.

[0086] On the basis of the travel times Δτ_(i), it is possible to deducetime offset values to be applied to the acoustic signals emitted by thevarious high-frequency transducers T1-T8, so that all of the acousticwave impulses emitted by said high-frequency transducers arrive at thebubble 5 at the same time. Thus, to make all of the acoustic impulsesfrom the transducers T1-T8 arrive at the gas bubble an instant t0, saidimpulses are emitted at respective instants t0-Δτi by the varioustransducers Ti. In other words, it is possible to focus the acousticwave impulses emitted by the transducers T1-T8 on the bubble 5, at thespecific position occupied by said bubble 5.

[0087] In addition, the arrival instant t0 at which the wave impulsesarrive at the gas bubble 5 is determined so as to coincide with thebubble being in a contraction stage.

[0088] This synchronization may be performed by detecting the instantst1 at which the light flashes are emitted by the gas bubble 5 under theeffect of the standing acoustic wave. The flashes are emitted veryregularly so that, once an instant t1 has been determined, it is knownthat the subsequent flash emissions will take place at instants t1+n.T,where n is a natural integer, and T is the period of the standingacoustic wave. In addition, the curve D(t) giving the diameter of thegas bubble 5 as a function of time is itself entirely known andrepetitive so that knowledge of the instant t1 makes it possible to knowsaid curve. it is thus possible to determine the instant t0 at which itis desired for the compression wave impulses emitted by the transducersT1-T8 to arrive simultaneously on the bubble 5.

[0089] Advantageously, said instant t0 may be chosen to correspond to amaximum of the curve D(t) shown in FIG. 5.

[0090] Since the duration θ of the compression ultrasound impulse isknown (e.g. about 700 nanoseconds (ns)), it is also possible to choosethe instant t0 so that the signal S2 generates compression of the bubble5 for most of the contraction stage 12 of the bubble and so that theincrease in pressure generated in this way continues at least until thelight emission instant t1.

[0091] Once the instant t0 has been determined, it is possible, at therespective instants t0-Δτ_(i), to trigger firing of compressionultrasound impulses S2 by the various transducers T1-T8, with a secondamplitude that is very much larger than the first amplitude. Forexample, the total amplitude of the pressure variation in the liquid inthe vicinity of the bubble 5 may then be about 8 bars, or even muchlarger.

[0092] By focusing the wave impulses emitted in this way, the gas bubble5 is not destroyed. In addition, a considerable increase is observed inthe intensity of the light emitted by the gas bubble 5 at the moment atwhich the compression ultrasound impulse is applied. By way of example,for a compression ultrasound impulse amplitude of about 8 bars in theliquid in the vicinity of the gas bubble 5, the intensity of the lightemitted by sonoluminescence is observed to increase by a factor of 2relative to the intensity emitted in the presence of the standingacoustic wave alone.

[0093] It should be noted that it is possible to increase the energyproduced by the gas bubble 5 by sonoluminescence even further byincreasing the amplitude of the compression ultrasound impulse, whichcan be obtained by increasing the power of the high-frequencytransducers T1-T8 and/or the number of said transducers.

[0094] In addition, it is possible to increase the energy emitted bysonoluminescence even further by causing the compression ultrasoundimpulse S2 to be preceded immediately by an expansion ultrasound impulseS3, i.e. a sudden drop in pressure to below the static pressure P0prevailing in the reservoir 2 (see FIG. 5) during the expansion stage 10of the gas bubble 5, which stage immediately precedes the contractionstage 12 during which the compression ultrasound impulse S2 is applied.Naturally, the impulses S2 and S3 may be emitted by the same transducersT1-T8.

[0095] It should be noted that, the transducers T′1, T′2 may optionallybe omitted, the transducers T1-T8 then serving both:

[0096] to generate a standing wave which holds the gas bubble 5 inposition and causes its diameter to vary cyclically; and

[0097] to generate compression and/or expansion wave impulses.

[0098] In addition, it should also be noted that it is possible tostabilize the gas bubble 5 elsewhere than at the center of the reservoir2, e.g. in the vicinity of its walls, by using a geometrical shape ofthe reservoir 2 and/or waveforms suitable for generating acousticantinodes at the desired location.

[0099] It is also possible to stabilize a plurality of gas bubbles 5 inthe reservoir 2, also by choosing a suitable geometrical shape for thereservoir 2 and/or suitable waveforms. In which case the transducersT1-T8 may be controlled so that they focus successively and/orsimultaneously onto the various gas bubbles 5.

[0100] Finally, it is optionally possible to replace the compressionwave impulses S2 entirely with the above-mentioned expansion waveimpulses S3, synchronized with the deformation cycle of the gas bubble 5so as to reach said gas bubble during an expansion stage.

1/ A method of generating photons by sonoluminescence, said methodcomprising at least the following steps: (a) generating at least onestanding acoustic wave (S1) in a liquid reservoir (2), said standingacoustic wave having at least one antinode; (b) trapping at least onegas bubble (5) in the liquid at said antinode of the standing acousticwave, said gas bubble then being subjected to a periodic deformationcycle comprising expansion stages (10) and contraction stages (12) inalternation; and (e) generating acoustic wave impulses (S2) in theliquid, which compression acoustic wave impulses are superposed on thestanding acoustic wave (S1) , and cause photons to be emitted by the gasbubble, by sonoluminescence; said method being characterized in that theacoustic wave impulses (S2) are caused to be emitted by a number n atleast equal to 2 of impulse firing transducers (T1-T8) disposed aroundthe gas bubble (5); in that a focusing training step (c) and asynchronization step (d) are interposed between the steps (b) and (e);and in that, during the focusing training step (c), the impulse firingtransducers are caused to emit acoustic wave impulses (S2) with a firstamplitude that is sufficiently small to avoid disturbing significantlythe position and the deformation cycle of the gas bubble (5), acousticsignals generated by said acoustic wave impulses in the liquid reservoirare measured, and time offsets are deduced therefrom to be applied torespective ones of the acoustic wave impulses generated by the variousimpulse firing transducers (T1-T8) so as to focus said acoustic waveimpulses onto the gas bubble (5) ; in that, during the step (d),instants at which acoustic wave impulses (S2) are emitted by the variousimpulse firing transducers (T1-T8) are determined so that each waveimpulse (S2) generated by the impulse firing transducers reaches the gasbubble (5) either during a contraction stage if the wave impulse is acompression wave, or during an expansion stage if the wave impulse is anexpansion wave; and in that, during the step (e), the impulse firingtransducers (T1-T8) are caused to generate the acoustic wave impulses atthe respective emit instants determined at the step (d), with a secondamplitude that is larger than the first amplitude. 2/ A method accordingto claim 1, in which the step (c) comprises the following sub-steps:(c1) each impulse firing transducer (T1-T8) is caused to emit anacoustic wave impulse (S2) in succession, with said first amplitude;(c2) after each acoustic wave impulse emission, each impulse firingtransducer (T1-T8) is caused to measure the acoustic signals s3_(ij)(t)generated by said acoustic wave impulse propagating in the liquidreservoir (2) , and said measured signals are stored, i and j beingindices respectively designating the impulse firing transducer thatemitted the acoustic wave impulse and the impulse firing transducer thatreceived the acoustic wave impulse corresponding to each measured signals3_(ij)(t) ; and (c3) at least on the basis of said measured signalss3_(ij)(t), said time offsets to be applied to respective ones of theacoustic wave impulses generated by the various impulse firingtransducers (T1-T8) are determined so as to focus said acoustic waveimpulses onto the gas bubble (5). 3/ A method according to claim 2, inwhich, during the sub-step (c3), travel times taken by the acoustic waveimpulses to travel between each impulse firing transducer (T1-T8) andthe gas bubble are determined, and said time offsets to be applied torespective ones of the acoustic waves generated by the various impulsefiring transducers so as to focus said acoustic wave impulses onto thegas bubble are deduced from said travel times. 4/ A method according toclaim 2 or claim 3, in which a preliminary calibration step (a0) isperformed, at least before the step (b), said calibration stepcomprising the following sub-steps: (a01) each impulse firing transducer(T1-T8) is caused to emit an acoustic wave impulse (S2) in succession,with said first amplitude; (a02) after each acoustic wave impulseemission, each impulse firing transducer (T1-T8) is caused to measureacoustic signals s1_(ij)(t) generated by said acoustic wave impulsepropagating in the liquid reservoir (2), and said measured signalss1_(ij)(t) are stored; during step (c) each impulse firing transducer(T1-T8) is caused to listen to the acoustic signals s2_(j)(t) receivedwhile the standing acoustic wave is being emitted in the presence of thegas bubble (5); and during the sub-step (c3), corrected signalss_(ij)(t)=s3_(ij)(t)−s1_(ij)(t)−s2_(j)(t) are calculated, and then saidtime offsets are determined on the basis of said corrected signals. 5/ Amethod according to claim 4, in which said time offsets are determinedby cross-correlation between said corrected signals. 6/ A methodaccording to any preceding claim, in which n is at least equal to
 8. 7/A method according to any preceding claim, in which the acoustic waveimpulses (S2) are compression acoustic wave impulses, and, during thestep (d), emit instants are determined at which the compression acousticwave impulses (S2) are emitted by the various impulse firing transducers(T1-T8) so that each compression acoustic wave impulse (S2) generated bythe impulse firing transducers reaches the gas bubble (5) during acontraction stage. 8/ A method according to claim 7, in which, duringstep (d), emission of the compression acoustic wave impulses by thevarious impulse firing transducers is synchronized with the deformationcycle followed by the gas bubble so that said compression acoustic waveimpulses generate an increase in the pressure of the liquid surroundingthe gas bubble at least until the end of said contraction stage. 9/ Amethod according to any one of claims 7 and 8, in which, during step(d), emission of the compression acoustic wave impulses by the variousimpulse firing transducers (T1-T8) is synchronized with the deformationcycle followed by the gas bubble, so that each compression acoustic waveimpulse generated by the impulse firing transducers reaches the gasbubble (5) substantially when said gas bubble has its maximum diameter.10/ A method according to any one of claims 7 to 9, in which thecompression acoustic waves (S2) generate acoustic vibration of amplitudeat least equal to 8 bars in the liquid in the vicinity of the gas bubble(5). 11/ A method according to any one of claims 7 to 10, in which,during step (e) , the compression acoustic wave impulse (S2) coming fromeach impulse firing transducer (T1-T8) is caused to be precededimmediately by an expansion acoustic wave impulse (S3) which is adaptedto reach the gas bubble (5) during the expansion stage (10) precedingthe contraction stage (12) during which said gas bubble receives thecompression acoustic wave impulses. 12/ A method according to anypreceding claim, in which the standing acoustic wave (S1) is caused tobe generated by at least two standing wave generation transducers (T′1,T′2) distinct from the impulse firing transducers (T1-T8). 13/ A methodaccording to any preceding claim, in which the standing acoustic wave(S1) is an ultrasound wave of frequency lying in the range 20 kHz to 30kHz and of amplitude in the vicinity of 1.3 bars.